The invention is an advanced method for controlling a solid catalyst alkylation process using Raman spectroscopy.
Alkylate is the alkane product mixture of the alkylation reaction of an alkene and an alkane. The alkene typically has from 2 to 6 carbon atoms, but may have as many as 20 carbon atoms. The alkane typically has from 4 to 6 carbon atoms. One important use of an alkylate is as a component in motor fuel, and its importance continues to grow with strict government regulations on historical octane number boosters such as lead anti-knock additives and aromatics. In order to boost the octane number of the motor fuel octane, it is desirable for the alkylate to have a high octane number, and, therefore, branched and multi-branched alkenes such as trimethylpentanes are the preferred components of the alkylate. Alkylation processes require a strong acid catalyst such as sulfuric acid or liquid hydrogen fluoride. More recently, due to environmental pressure, solid alkylation catalysts have been developed (see U.S. Pat. No. 2,999,074). Other alkylation reactions may involve the reaction of the alkene with an aromatic hydrocarbon, the most common of which is benzene.
Common solid catalysts used in alkylation processes have the drawback of rapid deactivation and require frequent regeneration. Once the catalyst begins to deactivate, its activity falls off almost exponentially. If the catalyst is not removed or regenerated, the conversion of the alkylation process significantly and quickly decreases, and the reactants contaminate the alkylate. Such contamination is to be avoided since feed material is wasted and, furthermore, it may be difficult to remove the reactants from other reactor effluent components. Also, it is beneficial to curtail catalyst deactivation early to prevent severe degradation of the catalyst.
In general, measuring the composition of the effluent of a reactor during the course of a chemical reaction is a common way to monitor the reaction occurring in a reactor. Many processes are conducted over a period of time and the resulting compounds in a reactor effluent may vary over the course of the chemical reaction. For example, when a reaction is initiated in a reactor, the concentration of a particular material may be at an initial level. As the process stabilizes the concentration of the material may undergo dramatic changes and then reach a point where changes in the concentration of the material are slow or gradual. Subsequent rapid changes in the concentration of the material may indicate a problem with the reaction such as rapid deactivation of the catalyst or a process upset. Control of process parameters may be based on changes in the composition of a reactor effluent during the course of the chemical reaction. For example, gradual decreases in the concentration of the desired product in a reactor effluent may indicate catalyst aging and may trigger periodic adjustments to the operating temperature of the reactor in order to increase, or reduce the rate of decrease, of the desired component in the effluent. Therefore, during the course of a chemical reaction, the concentration changes occurring as a result of the chemical reaction are often monitored by measuring the effluent of the reactor, see for example, U.S. Pat. Nos. 5,712,481 and 5,684,580.
In alkylation processes, monitoring the concentration of the alkene in the alkylate exiting the reactor is a common way to detect the deactivation of the catalyst. Gas chromatography has been used to measure the concentration of alkene in the alkylate. As the catalyst deactivates, the concentration of alkene in the alkylate increases and, at a particular stage of deactivation, the catalyst needs to be regenerated to prevent excessive alkenes in the product alkylate. In a commercial test, the lifetime of a catalyst can be as short as one minute, but is generally between about one minute and about five minutes. Even when operating on-line, the gas chromatographic analysis typically requires at least thirty minutes and during those thirty minutes the alkylate may contain large amounts of unconverted alkenes and the catalyst may become seriously deactivated.
As compared to control methods based on monitoring reactor effluent, the present invention provides advanced control through measuring the alkene concentrations by Raman spectroscopy at multiple locations within the alkylation process including at least two locations within the reaction zone(s) as well as at the alkylate product stream. The virtually instantaneous results allow for the adjustment of operating parameters or regeneration of catalyst before the alkylate is unacceptably contaminated with reactants, Furthermore, different operating parameters are adjusted depending upon the alkene concentration measured at different locations thus pairing a particular operating parameter with an ideal location for alkene measurement and control. Lastly, Raman spectroscopy is uniquely suited for use in alkylation processes since the other non-alkene components typically found in alkylation processes have weak Raman effects and little fluorescence.
The purpose of the invention is to provide an advanced method for controlling a solid catalyst alkylation process. At multiple locations throughout the process, including multiple locations within the reaction zone, on-line Raman spectroscopy is used to measure the concentration of alkenes. For example, an embodiment of the invention is one requiring measuring the Raman spectrum on-line over wave numbers from about 150 cmxe2x88x921 to about 1850 cmxe2x88x921 of: the reaction mixture in the reaction zone at a location near a feed input location, the reaction mixture in the reaction zone at a location downstream of the feed input location, and the alkylate. The concentration of alkene is then determined in: the reaction mixture at the location near the feed input location, A, the reaction mixture at the location downstream of the feed input location, B, and the alkylate, C, using the Raman spectra and a first, second, and third algorithm. The concentration, A, is compared with a predetermined range of desired concentrations of alkene in the reaction mixture at the location near the feed input location, D, and adjustment made, within established alkane feed stream flow rate control limits, when A is not within C, to the flow rate of the alkane feed stream according to a fourth algorithm to cause A to fall within D. The conversion of alkene occurring between the location near the feed input location and the location downstream of the feed input location, E, is determined by difference between A and B, and E is compared with a predetermined range of desired alkene conversion values, F, and adjustment made, within established alkene feed stream flow rate control limits and reactor temperature control limits, when E is not within F, to an operating parameter selected from the group consisting of flow rate of the alkene feed stream, reactor temperature, and a combination thereof, according to a fifth algorithm to cause E to fall within F. C is compared with a predetermined range of desired concentrations of alkene in the alkylate, G, and adjustment made, when C is not within G, to an operating parameter selected from the group consisting of severity of catalyst regeneration conditions, treatment of the feed streams, frequency of catalyst regeneration, rate of catalyst regeneration, and a combination thereof, according to a sixth algorithm to cause C to fall within G and to reset operating parameters selected from the group consisting of the flow rate of the alkane feed stream, the flow rate of the alkene feed stream, the reactor temperature, and a combination thereof, to within their respective established control limits.
Another embodiment of the invention is one where the alkylation process contains at least two serially-connected sub-reaction zones zi where i is an integer from 2 to n, each having an independent alkene feed stream and an alkane feed stream. In this embodiment, the Raman spectrum is measured on-line over wave numbers from about 150 cmxe2x88x921 to about 1850 cmxe2x88x921, of: the reaction mixture in each sub-reaction-zone zi at a location near the feed input location of that sub-reaction-zone zi the reaction mixture in each sub-reaction-zone zi at a location downstream of the feed input location of that sub-reaction-zone zi and the alkylate. The concentration of alkene in: the reaction mixture in each sub-reaction-zone zi at the location near the feed input location of each sub-reaction-zone zi, Azi, the reaction mixture in each sub-reaction-zone zi at the location downstream of the feed input location of each sub-reaction-zone zi, Bzi, and at the alkylate, C, is determined using the Raman spectra and a first, second, and third algorithm. Azi is compared with a predetermined range of desired concentrations of alkene in the reaction mixture of the sub-reaction-zone zi at the location near the feed input location of the sub-reaction-zone zi, Dzp and adjustment made, within established alkane feed stream flow rate control limits for sub-reaction-zone zi when Azi is not within Dzi to the flow rate of the alkane feed stream to sub-reaction-zone zi according to a fourth algorithm to cause Azi to fall within Dzi. The conversion of alkene occurring between the location near the feed input location and the location downstream of the feed input location of each sub-reaction-zone zi, Ezi, is determined from the difference between Azi and Bzi, and Ezi is compared with a predetermined range of desired alkene conversion values for each sub-reaction-zone zi, Fzp, and adjustment made, within established alkene flow rate control limits for sub-reaction-zone zi and temperature control limits for sub-reaction-zone zi, when Ezi is not within Fzp, to an operating parameter selected from the group consisting of flow rate of the alkene feed stream to sub-reaction-zone zi, temperature of sub-reaction-zone zi, and a combination thereof, according to a fifth algorithm to cause Ezi to fall within Fzi. C is compared with a predetermined range of desired concentrations of alkene in the alkylate, G, and adjustment made, when C is not within G, to an operating parameter selected from the group consisting of severity of catalyst regeneration conditions, treatment of the feed streams, frequency of catalyst regeneration, rate of catalyst regeneration, and a combination thereof, according to a sixth algorithm to cause C to fall within G and to reset operating parameters selected from the group consisting of the flow rate of the alkane feed streams to the sub-reaction zones, the flow rate of the alkene feed streams to the sub-reaction-zones, the temperatures of the sub-reaction zones, and a combination thereof, to within their respective established control limits.